Pb understanding how Incremental imaging works.

[MerleOne]

Hi,
I am in the following situation and I don't understand why the incrementals I create are so big :

I have a full image of my C: F:\Backup\R-Drive\MyPC-C-20111206Full.arc ~ 7.5 GB

I have created diff images using the GUI :
F:\Backup\R-Drive\MyPC-C-20111206Full1.arc ~3 GB
F:\Backup\R-Drive\MyPC-C-20111206Full2.arc ~3.5 GB
F:\Backup\R-Drive\MyPC-C-20111206Full3.arc ~4 GB
F:\Backup\R-Drive\MyPC-C-20111206Full5.arc ~5 GB

Now, I have built a script to create an incremental image :
append /a /o -append-inc="true" -s="1:2" -a="F:\Backup\R-Drive\MyPC-C-20111206Full.arc" -u -s-xr -s-n

According to the documentation, the incremental image file should contain changes done between MyPC-C-20111206Full5.arc creation and now, but it is almost as big as 5GB. And subsequent incrementals using the same script should be much smaller, but they aren't.

Am I missing something ?
Thanks for your help.

Thanks.